1. Field of the Invention
The present invention relates to a laser diode module (LD module) and a method for fabricating the same.
In a general optical communications system to which intensity modulation/direct detection (IM/DD) is applied, it is adapted, on the principle that the optical output power of a laser diode (LD) varies with the changes in the injection current thereto, such that intensity modulated light is obtained by supplying a modulating current pulse to an LD which is current-biased to the value close to its laising threshold value. Here, since the I-L characteristic (the characteristic indicating the relationship between the injection current and the optical output power) of a laser diode varies with its temperature, in order to obtain a constant operating condition independent of the ambient temperature, it becomes necessary to drive the LD whose temperature is controlled to remain constant or to carry out temperature compensation such that a constant optical power may be obtained independently of changes in the temperature of the LD. However, if it is taken into consideration that it is not easy to determine the temperature-dependent changes of the I-L characteristic, achieving accurate temperature compensation is practically difficult, and relying on the temperature compensation only is not favorable when deterioration of the LD is considered. Accordingly, in order to improve the reliability on the LD and eliminate the need for a temperature compensation circuit, it is required to achieve temperature control of the LD itself.
When fabricating an LD module provided with such temperature control, various parts such as the LD chip and a thermistor (thermally sensitive device) have to be fixedly mounted on a carrier with a special joining technique. Therefore, when any of the parts is found faulty after all the parts have been mounted, it is often the case that exchanging only the broken part is impossible. Accordingly, the yield rate of LD modules is not always high. Therefore, there is a demand for an increased yield rate in the fabrication of the LD modules.
2. Description of the Related Art
Referring to FIG. 13, there is shown a structure of the main portion of a conventional LD module in which it is adapted such that the resistance value of a thermistor 206 fixed on to a carrier 204 together with an LD chip 202 is detected and the drive current of an electronic cooling element 208 in contact with the carrier 204 is controlled so that the resistance value may be kept constant. Incidentally, the optical system for coupling the light output from the LD chip 202 to an optical fiber is not shown. Reference numeral 210 denotes a terminal for connecting the thermistor 206 with an external circuit, 212 denotes a terminal for connecting the electronic cooling element 208 with an external circuit, and 214 denotes a terminal for connecting the LD chip 202 with an external circuit.
When fabricating such a conventional LD module as shown in FIG. 13, an Au/Sn alloy is usually used for joining the LD chip and thermistor to the carrier. Therefore, when such trouble occurs that the thermistor gets broken after the LD chip and thermistor have been mounted on a carrier, it is impossible to melt the alloy for removing only the thermistor to exchange it with a good one and, hence, the module becomes a condemned goods. At this time, the most expensive LD chip becomes useless and, thus, the conventional arrangement was not suitable for improving the yield rate in the fabrication of the modules.
On the other hand, in conventional LD modules, the connections between the terminals for external connection and the thermistor and the like are usually provided by bonding wires made of gold having good heat conductivity. Hence, according to the temperature difference between the module and the surroundings, heat flows into the thermistor from the outside through the terminal and bonding wire or heat flows out of the thermistor to the outside through the bonding wire and terminal and, thus, there has been a problem that accurate temperature control has not been achievable.
Referring to FIG. 14, reference numeral 216 indicates the I-L characteristics of the module when the internal temperature and the external temperature are equal and I.sub.th indicates the laising threshold current. When the ambient temperature of the module becomes relatively high, heat flows into the thermistor through the terminal and bonding wire to make the temperature of the thermistor higher than that of the carrier and LD chip. As a result, the LD chip is controlled to a lower temperature than the right temperature and the I-L characteristic is shifted to the left as indicated by 218 in FIG. 14 and the threshold current I.sub.th also decreases. When, on the other hand, the ambient temperature of the module becomes relatively low, heat flows from the thermistor to the outside through the bonding wire and terminal and the temperature of the thermistor becomes lower than the temperature of the LD chip and carrier. As a result, the LD chip is controlled to a relatively high temperature and the I-L characteristic makes a parallel translation to the right as indicated by 220 in FIG. 14. The variation in the oscillating threshold current I.sub.th due to such change in the ambient temperature is 2-3 mA and this variation cannot be neglected in an LD module applied to a high-speed system (for example, at 1.8 Gb/s).